The invention relates to a procedure for determining optimum ignition times with regard to engine operation, especially for motor vehicles, on the basis of a basic set of performance characteristics, which were previously determined on an engine test-stand by reference to several test examples and which, under certain operating conditions, indicate optimum ignition times. During engine operation this set of performance characteristics, which is stored in an arithmetic computing unit, is called up for the purposes of setting the most favorable ignition time at any particular moment, while taking further values into consideration which are specific to the motor vehicle. These vehicle-specific values include the knock limit under particular conditions of operation of the vehicle.
In vehicle engines, low fuel consumption and the emission of clean exhaust gas are becoming ever more important requirements of engine operation. One method by which these requirements can be met comprises setting and maintaining ignition times on the engine to values which are optimum with regard to the instantaneously-changing operating conditions as they occur, this method being implemented with the aid of an electronic ignition system. However, at the same time, this means that the engine should be run as close beneath the knock limit as possible, since the ignition times which are optimum with regard to fuel consumption lie on this limit. However, in doing so, it should be ensured, as far as possible, that the knock limit is not exceeded, since there is a very great risk of damaging the engine if the knock limit is exceeded too frequently.
Since the knock limit does not, however, represent a constant limit during the long-term operation of an engine, sampling of the knock limit at definite intervals is unavoidable if its trend is to be known.
In a known procedure for determining optimum ignition times with respect to engine operation, the method followed is to use a basic set of performance characteristics which were previously assembled on the test-stand by reference to several test examples, this set of characteristics then being used to determine the ignition times for the engine. When a particular operating condition is dominant, the associated ignition time (ignition angle) is taken from the basic set of performance characteristics, and a check is made, at the same time, with the aid of a knock sensor, as to whether the engine is operating above or below the knock limit. If, together with an ignition-time value, a knock signal is generated, the ignition times are immediately adjusted, with the aid of the computing unit, to provide a certain separation of engine operation beneath the knock limit, from which point the ignition times are again gradually shifted towards the knock limit. When a knock signal occurs again, the ignition time is once again retarded, so that the knock limit is approached at regular intervals, whereupon the engine must repeatedly be forced to run with detonating combustion. This is very disadvantageous, on account of the risk associated therewith that the engine will be damaged.